In the copolymerization of 1,3-butadiene and isoprene with unmodified neodymium catalysts, the 1,3-butadiene polymerizes about 19 times faster than the isoprene. For this reason, such copolymers do not have a random distribution of monomers. One end of the polymer chains contain mostly repeat units which are derived from butadiene (which polymerized faster) and the other end of the polymer chains contain mostly repeat units which are derived from isoprene (which polymerized slower). As the polymerization proceeds, the availability of butadiene monomer for polymerization diminishes leaving more and more isoprene to polymerize subsequently. This causes such isoprene-butadiene rubbers to be tapered.
U.S. Pat. No. 4,663,405 discloses that conjugated diolefin monomers can be polymerized with a catalyst system which is comprised of (1) an organoaluminum compound, (2) an organometallic compound which contains a metal from Group III-B of the Periodic System, such as lanthanides and actinides, and (3) at least one compound which contains at least one labile halogen atom. U.S. Pat. No. 4,663,405 also discloses that the molecular weight of the polymers made with such catalyst systems can be reduced by conducting the polymerization in the presence of a vinyl halide. However, its teachings do not specifically disclose copolymerizations of isoprene with butadiene and do not provide any technique for making the isoprene monomer polymerize at a rate that is similar to that of the butadiene monomer. Thus, its teachings do not provide a technique for synthesizing random, non-tapered isoprene-butadiene rubbers with catalyst systems which are comprised of (1) an organoaluminum compound, (2) an organometallic compound which contains a metal from Group III-B of the Periodic System, such as lanthanides and actinides, and (3) at least one compound which contains at least one labile halogen atom.
U.S. Pat. No. 5,405,815 discloses a process or preparing a catalyst system which is particularly useful for copolymerizing isoprene and 1,3-butadiene monomers into rubbers which comprises the sequential steps of (1) mixing (a) an organoaluminum hydride, (b) a member selected from the group consisting of aliphatic alcohols, cycloaliphatic alcohols, aliphatic thiols, cycloaliphatic thiols, trialkyl silanols, and triaryl silanols, and (c) optionally, 1,3-butadiene in an organic solvent to produce a modified organoaluminum catalyst component; (2) adding an organometallic compound which contains a metal from Group III-B of the Periodic System to the modified organoaluminum catalyst component to produce a Group III-B metal containing catalyst component; (3) adding a compound which contains at least one labile halogen atom to the Group III-B metal containing catalyst component; and (4) aging the catalyst system after the compound which contains at least one labile halogen atom is added to the modified Group III-B metal containing catalyst component for a period of 10 minutes to 6 hours, wherein the catalyst system is aged at a temperature which is within the range of about 30° C. to about 85° C.